Interstellar Medium

This set of notes by Nick Strobel covers: the interstellar medium--the effect of dust, emission nebulae, 21 cm radiation, mapping galactic structure, and molecules.These notes will be in outline form to aid in distinguishing various concepts. As a way to condense the text down I'll often use phrases instead of complete sentences. The vocabulary terms are italicized.


Interstellar Medium (ISM)


Stuff between the stars. 10-15% of the visible mass of the Galaxy. 99% of the ISM mass is gas; 1% dust. ``So what?'' Why do we worry about the interstellar medium? The interstellar medium affects starlight and stars are formed from ISM!

A. Dust

Dust-about the size of the wavelength blue light or smaller. Water ice, graphite (Carbon), Silicon in highly flattened flakes or needles. Effects of dust on light:


  1. Extinction--dimming of starlight at all wavelengths. In 1930 R.J. Trumpler plots angular diameter of clusters vs. distance to cluster. Distance found from inverse square law of brightness. IF clusters all have nearly same linear diameter s, then the angular diameter should equal a constant size / distance (theta = s/D). But he found a systematic increase of the linear size of the clusters with distance. Unreasonable! It would mean that nature had put the Sun at a special place where the size of the clusters was the smallest. More reasonable: the Sun is in a typical spot. It's simply that more distant clusters have more stuff between us and cluster so that they appear fainter (farther away) than they really are. Extinction caused by scattering of light out of the line-of-sight--less light reaches us.



  2. Reddening--extinction depends on wavelength: amount of extinction is approximately a constant / wavelength (E ~ 1/lambda). Bluer wavelengths scattered more than redder wavelengths. 1/lambda behavior says that the dust size must be about the wavelength of light (on the order of 10^-5 cm). Less blue light reaches us so object appears redder than it should. Trumpler showed that a given spectral type of star becomes increasingly redder with distance. Same sort of process at work to make the Sun appear redder when it is close to the horizon. Dust and gas molecules in the air scatters out the bluer colors of sunlight from your line-of-sight. When Sun close to horizon, this effect is enhanced (light is going through more air).

B. Gas


About 90% Hydrogen, 10% Helium, and traces of other elements. Observe Hydrogen in ionized, neutral atomic, and molecular forms. At visible wavelengths dust has greater effect on light than gas. Looking at spectral lines of binary, see narrow lines that do not move and other broader lines shifting as stars orbit each other. The narrow lines are from gas in the ISM.

    H II Regions

  1. H II Regions--fluorescence of hydrogen atoms. Ultraviolet light from hot O & B stars absorbed by the Hydrogen gas and re-emitted mostly at visible wavelengths, primarily 6563 Å(red color). Each UV photon produces a visible photon. O & B stars only found in regions of star formation (know why?). H II region spectra much simpler than star spectra-easier to decipher. Stuff making stars is mostly Hydrogen and Helium. Distribution of H II regions is in spiral pattern. O & B are spiral tracers also. The ``II'' of H II means that Hydrogen is missing one electron. A He III nebula would be Helium gas with two missing electrons. A H I nebula would be neutral atomic Hydrogen.

    21 cm


  2. 21 cm--emission line at wavelength 21 cm produced by neutral atomic Hydrogen. In 1944 van de Hulst predicts it. Hydrogen in space is in ground state. Electron moving around proton has spin parallel (same direction) as proton or opposite (anti-parallel). Anti-parallel is slightly lower energy state. Remember that atoms always want to be in the lowest energy state possible. Sometimes Hydrogen atoms collide and the spins are re-aligned. Eventually (on average few million years) the electron flips its spin to get in lowest energy state. Low energy photon (frequency 1420.4 MHz which is at wavelength 21.1 cm) emitted. This is a RARE transition, but there is lots of Hydrogen in space! Enough to make noticeable amount of 21 cm radiation. Milky Way (our galaxy) has about 3 billion solar masses of H I gas with about 70% of that further out in galaxy from the Sun. Most of the H I gas in disk is located within 220 pc from the midplane of the disk. What's very nice is that 21 cm radiation is not blocked by dust!!
    Find density of atomic Hydrogen along line of sight from intensity of 21 cm line.
    Rotation curve--plot of rotation speed vs. distance from Galaxy center. We assume that the gas clouds move in the plane of the disk on circular orbits. Jan Oort 1927 finds stars closer to center complete greater fraction of their orbit in a given time than stars farther out from center--differential rotation (different angular speeds). Look at Doppler velocities of Hydrogen gas along different lines of sight. Maximum doppler velocity at distance = (solar distance) x sin(galactic longitude). Observe maximum doppler velocity along different lines of sight to get rotation curve.
    Map out Galaxy's structure. 21 cm line profile has several Doppler-shifted peaks that are narrow and well-defined. IF the rotation curve is already known, then we can use the doppler speed of peaks to get the distance to the Hydrogen producing each peak. Use intensity of peak to get density. Get spiral pattern in a thin disk for almost all of Galaxy! See picture below:



  3. Other molecules: H_2, CO, water OH, NH_3, SiO, CO_2, 100+ other molecules, many of which have Carbon in them (organic). Most of the molecules are H_2 and CO. Molecular clouds:
    Most of the molecules in the ISM are clumped together into clouds with masses anywhere from just a few solar masses to over a million solar masses with radii ranging from a few pc to over 100 pc. Milky Way has about 2.5 billion solar masses of molecular gas with about 70% of it in a ring at 4-8 kpc distance from the center. (The Sun is about 8 kpc from the galactic center.) Not much molecular gas at 1-3 kpc distance from center. About 15% of total molecular gas mass is located close to galactic center within 1.5 kpc from the center. Most of the H_2 gas is clumped in the spiral arms within the disk and stays within 120 pc of the disk midplane. Stars form in the molecular clouds.
    H_2 absorption lines detected in ultraviolet. However, gas and dust become so thick in a molecular cloud that ultraviolet extinction is too large to accurately measure all of the H_2 in the interior of the cloud. Fortunately, we see evidence of a correlation between amount of CO and H_2 so we use the easily detected CO radio emission lines (at 2.6 and 1.3 mm) to infer the amount of H_2. CO emission caused by H_2 molecules colliding with CO molecules. More H_2 means more collisions which means more CO emission.
    Is one gas cloud actually made of many smaller gas clouds? Some say that 90% of H_2 locked up in 5000 Giant Molecular Clouds with masses greater than 10^5 solar masses and diameters greater than 20 pc with the monster ones (diameters greater than 50 pc and having more than a million solar masses) making up 50% of the total mass. Others say the giants are actually made of smaller clouds.

Photo Gallery of Nebulae


The pictures come from various sites around the world. I have small ``thumbnail'' versions of the images on the page so the gallery page should load quickly. Links are provided to larger-scale versions. If you want to see some gorgeous pictures then go to the nebulae picture gallery. I will be adding more pictures and explanations in the near future.

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last updated 29 Nov 95

Nick Strobel -- Email:

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